The present invention relates to the field of low noise amplifier circuits (LNA) and, more specifically, to amplifiers used in radio frequency reception heads. The present invention more specifically applies to low noise amplifiers and to radio frequency reception heads intended for mobile telephony circuits likely to operate in two distinct frequency bands, for example, centered on 950-MHz and 1.85-GHz frequencies. Such mobile telephony systems are called double-band systems and the central frequencies associated with each band depend on the telecommunication standards. For example, for standards GSM and DCS, the reception bands (that transit through a head to which the present invention applies) are respectively included between 925-960 MHz and 1805-1880 MHz, with the transmission bands being respectively included between 880-915 MHz and 1710-1785 MHz. The use of double-band systems is linked to a need for increasing the capacity of mobile telephony networks.
FIG. 1 very schematically shows a conventional example of a double-band type radio frequency reception head 1. Head 1 receives a radio frequency signal RF that comes from a reception antenna (not shown) by transiting, possibly, through an antenna coupler and/or an isolation transformer (not shown). Most often, as will be seen hereafter, the radio frequency signal is of differential form. However, for simplification, FIG. 1 will be discussed in relation with a non-differential operation.
Signal RF is sent onto two low noise amplifiers 2, 3 (LNA1, LNA2), respectively associated with each central frequency of the system pass-bands. For example, amplifier 2 exhibits a maximum gain for a frequency on the order of 1850 MHz, while amplifier 3 exhibits a maximum gain for a frequency on the order of 950 MHz. Each amplifier 2, 3 is associated, at its output, with a filter 4, 5 (F1, F2) of band-pass type. Filters 4 and 5 are used to suppress the image frequencies of the respective central frequencies of the pass-bands. These filters are generally formed in so-called coplanar technology and are of surface wave type. In the above example, filter F1 is centered on the 950-MHz frequency, while filter F2 is centered on the 1850-MHz frequency. The respective outputs of filters 4 and 5 are sent onto first inputs of two multipliers 6, 7. The second respective inputs of multipliers 6 and 7 receive a frequency from a local oscillator OL1, OL2. The respective frequencies of local oscillators OL1 and OL2 are chosen so that, at the output of one of multipliers 6 and 7, the central frequency of signal FI corresponds, whatever the channel, to the intermediary frequency chosen for the radio frequency head. According to applications, the frequency of output signal FI of head 1 is the central frequency of the channel, or another arbitrary low frequency (for example, on the order of some hundred MHz, or even less). In applications to mobile telephony, the width of each channel is 200 kHz.
Amplifiers 2 and 3 and multipliers 6 and 7 are controlled by signals, respectively CTRL and NCTRL, having the function of selecting one of the two parallel paths of the radio frequency head according to the band in which the received channel is located.
FIG. 2 still very schematically shows a second example of a radio frequency reception head 1xe2x80x2. As in the example of FIG. 1, each frequency band is associated with a low noise amplifier, respectively 2, 3, the activation of which is obtained by a control signal, respectively CTRL and NCTRL. The essential difference between FIG. 1 and 2 is that, in FIG. 2, an image frequency reject mixer 10 is used. Such a mixer includes two input multipliers 11, 12 receiving, each, the signal coming from the operating amplifier 2 or 3 and the frequency provided by a local oscillator OL1 or OL2, respectively phase-shifted by 90xc2x0 for multiplier 11 and unshifted for multiplier 12. The selection of the local oscillator OL1 or OL2 to be used is effected by means of a switch Ko controlled, for example, by one of signals CTRL or NCTRL to select the local oscillator adapted to the amplifier 2 or 3 that is used. The respective outputs of multipliers 11 or 12 are individually sent, via selectors, respectively K1 and K2, onto phase-shifters by plus or minus 45xc2x0. Thus, multiplier 11 is associated with two phase-shifters 13 and 14, respectively by +45xc2x0 and by xe2x88x9245xc2x0, the respective inputs of which correspond to two output terminals of selector K1, the input of which is connected to the output of multiplier 11. Similarly, multiplier 12 is associated with two phase-shifters 15 and 16 respectively by +45xc2x0 and by xe2x88x9245xc2x0, the respective inputs of which are associated with two output terminals of selector K2, the input terminal of which is connected to the output of multiplier 12. The output terminals of phase-shifters 13 and 14 are connected to a first input of an adder 17 while the output terminals of phase-shifters 15 and 16 are connected to a second input of this adder 17, the output of adder 17 providing the signal at intermediary frequency FI. Of course, a single phase-shifter of each pair 13, 14 or 15, 16 is used according to the received radio frequency band. Further, the phase-shifters are used in opposition, that is, if the output of multiplier 11 is phase-shifted by +45xc2x0, the output of multiplier 12 is phase-shifted by xe2x88x9245xc2x0, and conversely. Selectors K1 and K2 are, for example, respectively controlled by signals CTRL and NCTRL to select the respective phase shifts to be brought according to the frequency of signal RF.
The operation of the double-band radio frequency heads such as illustrated in FIGS. 1 and 2 is perfectly well known and will not be detailed any further. It should only be noted that image frequency reject mixer systems are described in many publications, for example xe2x80x9cA 2.5 GHz BiCMOS image reject front endxe2x80x9d, by M. D. Mc Donald, ISSCC93, paper TP94, pp. 144-145, xe2x80x9cAn improved Image Reject Mixer and a Vco fully integrated in a BiCMOS processxe2x80x9d, by D. Pache, J. M. Fournier, G. Billot and P. Senn, in Proceed in Nomadic Microwave for Mobile Communications and Detection, Arcachon, November 1995, and xe2x80x9cAn improved 3 V 2 GHz BiCMOS Image Reject Mixer ICxe2x80x9d by D. Pache, J. M. Fournier, G. Billot and P. Senn, in Proceedings of CICC, May 1995, USA, the respective contents of which are incorporated herein by reference.
FIG. 3 very schematically shows the upstream portion of a radio frequency reception head 1, 1xe2x80x2 such as illustrated in FIGS. 1 and 2, that is, the portion located upstream of amplifiers 2, 3. In the example of FIG. 3, the case where the low noise amplifiers receive differential signals has been shown, which is most often the case. As illustrated in FIG. 3, an antenna 20 intercepts the radio frequency signal. This antenna is associated with the primary winding 21 of a transformer 22, the secondary winding of which has a midpoint, so that a first portion 23a provides signal RF while a second portion 23b provides inverted signal NRF. The midpoint of the secondary winding receives a voltage reference REF, for example the ground. Signals RF and NRF are each sent onto the two differential inputs of low noise amplifiers 2 and 3. Each amplifier 2, 3, has, similarly, two differential outputs towards filters 4 and 5 (FIG. 1) or mixer 10 (FIG. 2).
A disadvantage of conventional double-band radio frequency reception heads is that low noise amplifiers are particularly bulky. Accordingly, the use of two low noise amplifiers for each frequency of the double-band system adversely affects the system miniaturization, be it in terms of silicon surface for the integration of the radio frequency head, or in terms of number of input/output terminals, each frequency having its specific input. This in particular introduces two external matching networks. As illustrated in FIG. 4 hereafter, the input signals of low noise amplifiers are generally submitted to an impedance matching by means of external inductive and capacitive elements. The use of two low noise amplifiers proportionally increases the copper surface required to form the external inductive components.
FIG. 4 shows an example of a low noise amplifier circuit having a differential structure. Such an amplifier is based on the use of two input amplifiers, for example, bipolar transistors T1 and T2, each associated in series with two other amplifiers, for example bipolar transistors T3 and T4, to form two cascode assemblies. Each cascode assembly forming one of the branches of the differential circuit is associated with a trap circuit (circuit LC), respectively 30 and 31, assembled in series with one of amplifiers T3 and T4. Further, each amplifier T1, T2 is associated with a compensation inductance, respectively 32, 33, the function of which will be explained hereafter. LC circuits 30 and 31 are respectively formed of an inductance 34 in parallel with a capacitor 35 and of an inductance 36 in parallel with a capacitor 37. Inductances 36 and 34 have the same values, as well as capacitors 35 and 37.
A first terminal of inductances 34, 36 and of capacitors 35, 37 is connected to a terminal 38 of application of a supply voltage Vcc. The second respective terminals of inductances 34 and 36 and of capacitors 35 and 37 are, in the example of FIG. 4, connected to the collector of transistor T3 or T4 according to the LC circuit 30 or 31 to which they belong. The bases of transistors T3 and T4 are grounded while their emitters are connected to the respective collectors of transistors T1 and T2. The respective emitters of transistors T1 and T2 are connected to a first terminal of the compensation inductances 32 and 33 to which they are associated. The second respective terminals of inductances 32 and 33 are connected to a first terminal of a current source 39, the other terminal of which is grounded. The respective bases of transistors T1 and T2 receive input signals IN and NIN via matching inductances 40 and 41, the function of which will be discussed hereafter. Differential outputs OUT and NOUT of the low noise amplifier of FIG. 3 are defined by the respective collectors of transistors T3 and T4.
To obtain a low noise amplifier, that is, an amplifier having a strong rejection rate out of the desired pass-band and a strong gain for the central frequency of this pass-band, inductances 32, 33, 34, 36, 40 and 41, as well as capacitors 35 and 37, are matched accordingly.
Base inductances 40 and 41 and emitter inductances 32 and 33 of respective transistors T1 and T2 enable matching the input impedance of the amplifier. Inductances 32 and 33 enable matching with the capacitance of each input of the differential circuit, essentially formed by the base-emitter capacitance of each transistor T1 and T2. Inductances 40 and 41 enable matching with the base capacitance of transistors T1 and T2. Inductances 40 and 41 actually symbolize the inductances linked with the path of the input signals as well as with the inductances of the connection pads and of the package containing the integrated circuit. It should be noted that the more the input impedance of transistors T1 and T2 is inductive, the narrower the band.
The use of a differential structure enables obtaining a common mode rejection, while the cascode assemblies make the input impedance independent from the reactive loads formed by LC circuits 30 and 31. To obtain a low noise amplifier, the respective LC circuits must be resonant at the chosen value, that is, at the central frequency of the selected useful band. Therefore, each band of the reception head is associated with an LNA dedicated thereto.
An amplifier circuit such as illustrated in FIG. 4 is described, for example, in European patent application EP-A-0911969 of the applicant, the content of which is incorporated herein by reference.
In addition to problems linked with the space occupied by two low noise amplifiers, the needs for additional switches to select that of the amplifiers that is to operate can generate turn-on delays in the system due to switching delays. Further, the use of switches takes up additional space and requires control signals.
The embodiments of the present invention provide a novel radio frequency signal reception head only using a single low noise amplifier and being likely to be assigned to a double-band system.
Another embodiment of the present invention provides a low noise amplifier with two distinct pass-bands.
The embodiments of the present invention provide a solution that minimizes the use of switches to minimize the space taken up by the circuit in integrated form.
The embodiments of the present invention also provide a solution that is particularly well adapted to mobile telephony systems.
To achieve the foregoing, the embodiments of the present invention provide an amplifier circuit including at least one first input amplifier, at least one second amplifier cascode-assembled with the at least one first amplifier, at least one reactive impedance circuit, mounted in series with the at least one second amplifier, the at least one reactive impedance circuit being formed by two impedances respectively exhibiting a maximum value for a first and a second frequency, to form a double-band amplifier circuit.
According to an embodiment of the present invention, said impedances are associated in series and are sized to each exhibit a maximum value and a high quality factor on one of the two operating frequencies of the circuit.
According to an embodiment of the present invention, each impedance is formed by an inductance in parallel with a capacitor.
According to an embodiment of the present invention, the respective ratios between the inductance and the capacitor of each impedance range between 3,000 and 4,500.
According to an embodiment of the present invention, the inductances and capacitors of the impedances are sized so that the square root of the ratio between the inductance and the capacitance of the first impedance is greater than 1010 times the value of the inductance of the second impedance, and that the square root of the ratio between the impedance and the capacitance of the second impedance is greater than 0.5*1010 times the value of the inductance of the first impedance.
According to an embodiment of the present invention, the circuit includes two identical branches in parallel, each formed of a cascode assembly of a first and of a second amplifier, in series with a reactive impedance circuit.
According to an embodiment of the present invention, the respective impedances of the differential inputs of the amplifier circuit are sized to obtain a wide band matching covering the two operating frequency bands of the amplifier.
The present invention also provides a radio frequency signal reception head adapted to operating on two bands of different frequencies, including an amplifier circuit such as hereabove.